Overshoot protection circuit for LED lighting

ABSTRACT

For an LED lighting application, overshoot-protection circuitry prevents an LED controller, such as a matrix lighting controller (MLC) that controls an array of LEDs, from falsely detecting an open-circuit condition in an LED controlled by the LED controller, by limiting the magnitude of an overshoot voltage (due to long-wire parasitic inductances) from occurring when a switch in the LED controller that is used to control the LED is turned off.

BACKGROUND

The present invention relates to LED lighting and, more Particularly, toa circuit for controlling the operations of LED lighting.

FIG. 1 is a schematic block diagram a conventional LED lighting system100 comprising a set of 12 light-emitting diodes (LEDs) 102(1)-102(12)controlled by a matrix lighting controller (MLC) chip 110, where theLEDs 102 are connected to the MLC chip 110 via wires 104. The 12 LEDs102 are connected in series between a constant-current source 106 andground VSS. The voltage at the constant-current source 106 is 60V. Inaddition, each LED 102 is connected by a pair of wires 104 to adifferent transistor-based switch 112, where each switch 112 is in turncontrolled by a different pulse-width modulation (PWM) driver 114.

FIG. 2 is a schematic circuit diagram showing a portion of the LEDlighting system 100 of FIG. 1. In particular, FIG. 2 shows theconstant-current source 106, the LEDs 102(10)-102(12), and theircorresponding switches 112(10)-112(12) and wires 104, each of whichcontributes some amount of parasitic inductance. As shown in FIG. 2, theswitch 112(11) comprises an n-type main transistor MN1, an n-typeturn-off transistor MN2, and a turn-on switch S1, which is connected toa current source 113. Although not shown in FIG. 2, each of switches112(10) and 112(12) has an analogous set of elements.

The switch 112(11) is opened by (i) opening the turn-on switch S1 and(ii) turning on the turn-off transistor MN2, which drains the gate ofthe main transistor MN1 to turn off the main transistor MN1. The switch112(11) is closed by (i) turning off the turn-off transistor MN2 and(ii) closing the turn-on switch S1, which charges the gate of the maintransistor MN1 to turn on the main transistor MN1. When the switch112(11) is turned off, current from the constant-current source 106passes through the LED 102(11), causing the LED 102(11) to emit light.When the switch 112(11) is turned on, the LED 102(11) is shorted, andthe current instead passes through the switch 112(11), thereby bypassingthe LED 102(11), which as a result will not emit light.

In general, the MLC chip 110 selectively turns on and off the switches112(1)-112(12) to control the light emitted from the respective LEDs102(1)-102(12). Note that, if switch 112(11) is off and switch 112(12)is on, then current will flow from left to right through the upper wire104 u. If switch 112(11) is off and switch 112(10) is on, then currentwill flow from right to left through the lower wire 104 l. On the otherhand, if switch 112(11) is on and switch 112(12) is off, then currentwill flow from right to left through the upper wire 104 u. If switch112(11) is on and switch 112(10) is off, then current will flow fromleft to right through the lower wire 104 l.

As shown in FIG. 1, the MLC chip 110 also has short-circuit (SC)detection circuitry 116 and open-circuit (OC) detection circuitry 118designed to detect SC and OC conditions, respectively, in the individualLEDs 102 in order to detect faulty LEDs 102 and also to protect thecorresponding switches 112. For example, if the LED 102(11) fails withan OC condition, then, when the corresponding switch 112(11) is turnedoff, the voltage across the LED 102(11)—and therefore thedrain-to-source voltage Vds across the main transistor MN1—can get highenough to permanently damage the transistor MN1. To prevent that damage,the OC detection circuitry 118 is designed to quickly detect a highdrain-to-source voltage Vds indicative of an OC condition and takeappropriate measures, like quickly turning on the switch 112(11), inorder to inhibit the drain-to-source voltage Vds from getting largeenough to permanently damage the transistor MN1. During normaloperations, the MLC chip 110 controls the relative timing of turning onand off the different switches 112(1)-112(12) to avoid over-voltageconditions from occurring.

Unfortunately, if the wires 104 that connect the MLC chip 110 to theLEDs 102 are sufficiently long, then the wires 104 will contributeenough parasitic inductance to result in false-positive detections ofLED OC conditions by the OC detection circuitry 118. For example, ifswitch 112(11) is on and switches 112(10) and 112(12) are off, thencurrent will flow down through the LED 102(12), from right to leftthrough the upper wire 104 u, down through switch 112(11), from left toright through the lower wire 104 l, and down through the LED(10). Ifswitch 112(11) is then turned off, the parasitic inductances in theupper and lower wires 104 u and 104 l will resist an instantaneousdecrease in the current flowing through those wires, resulting in aninduced drain-to-source voltage Vds across the transistor MN1. Thelonger the wires 104, the greater the parasitic inductance in the wires104 and the greater the induced drain-to-source voltage Vds. In suchcases, the OC detection circuitry 118 may misinterpret a high Vds as anOC condition in the corresponding LED(11). Note that, even if switch112(10) and/or switch 112(12) is on, then other wires 104 willcontribute parasitic inductances that can lead to false-positivedetections of an OC condition in the LED(11).

FIG. 3 is a timing diagram showing certain signals associated with theoperation of the LED lighting system 100 of FIGS. 1 and 2 when afalse-positive detection of an OC condition occurs. At time t0, switchS1 is open and the turn-off transistor MN2 is on (i.e., the gate voltageG2 is high), which drives the gate voltage swg of the main transistorMN1 low and turns MN1 off. With MN1 off, the drain voltage swd of MN1 ishigh and the LED 102(11) is on. Thus, from time t0 to time t1, currentis flowing through the LED 102(11) and no current is flowing through theswitch 112(11).

At time t1, the gate voltage G2 is driven low to turn off the transistorMN2 and switch S1 is closed to drive the gate voltage swg high and turnon the transistor MN1, which drives the drain voltage swd of MN1 low andturns off the LED 102(11). Thus, from time t1 to time t2, no current isflowing through the LED 102(11) and instead current is flowing throughswitch 112(11) and, if switches 112(10) and 112(11) are both off, alsothrough the corresponding wires 104 u and 104 l connecting the switch112(11) to the LED 102(11).

At time t2, switch S1 is opened again and the gate voltage G2 of MN2 isagain driven high to turn on MN2, which drives the gate voltage swg ofMN1 low and turns off MN1. In this particular situation, the presence ofparasitic inductances in the wires 104 u and 104 l cause the drainvoltage swd of MN1 to rise and then oscillate in a damped manner. If theinitial rise of the drain voltage swd is large enough, then the OCdetection circuitry 118 will misinterpret the corresponding largedrain-to-source voltage Vds as indicating an OC condition for LED102(11).

It would be advantageous to provide circuitry that prevents suchfalse-positive detection of OC conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will become more fully apparent from thefollowing detailed description, the appended claims, and theaccompanying drawings in which like reference numerals identify similaror identical elements.

FIG. 1 is a schematic block diagram of a conventional LED lightingsystem;

FIG. 2 is a schematic circuit diagram showing a portion of the LEDlighting system of FIG. 1;

FIG. 3 is a timing diagram showing certain signals associated with theoperation of the LED lighting system of FIGS. 1 and 2 when afalse-positive detection of an OC condition occurs;

FIG. 4 is a schematic diagram of a portion of an LED lighting systemaccording to one embodiment of the present invention;

FIG. 5 is a timing diagram showing certain signals associated with theoperation of the LED lighting system of FIG. 4 for the same scenario asin FIG. 3; and

FIG. 6 is a schematic circuit diagram of overshoot-protection circuitryaccording to an alternative embodiment of the invention.

DETAILED DESCRIPTION

Detailed illustrative embodiments of the invention are disclosed herein.However, specific structural and functional details disclosed herein aremerely representative for purposes of describing example embodiments ofthe invention. The invention may be embodied in many alternate forms andshould not be construed as limited to only the embodiments set forthherein. Further, the terminology used herein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting of example embodiments of the invention.

As used herein, the singular forms “a,” “an, ” and “the,” are intendedto include the plural forms as well, unless the context clearlyindicates otherwise. It further will be understood that the terms“comprises,” “comprising,” “includes,” and/or “including,” specify thepresence of stated features, steps, or components, but do not precludethe presence or addition of one or more other features, steps, orcomponents. It also should be noted that in some alternativeimplementations, the functions/acts noted may occur out of the ordernoted in the figures. For example, two figures shown in succession mayin fact be executed substantially concurrently or may sometimes beexecuted in the reverse order, depending upon the functionality/actsinvolved.

In one embodiment, the present invention is an article of manufacturecomprising an overshoot-protection circuit for an LED (light-emittingdiode) controller. The overshoot-protection circuit comprises (i) aswitch-drain node configured to be connected to a drain of a switch inthe LED controller that controls at least one LED, (ii) a switch-gatenode configured to be connected to a gate of the switch in the LEDcontroller that controls the LED, and (iii) inter-node circuitryconnected between the switch-drain and switch-gate node and configuredto limit magnitude of drain-to-source voltage of the switch.

FIG. 4 is a schematic diagram of a portion of an LED lighting system 400according to one embodiment of the present invention that corresponds tothe same portion of the conventional LED lighting system 100 shown inFIG. 2. The circuitry shown in FIG. 4 is the same as the circuitry shownin FIG. 2, except that, in FIG. 4, the MLC chip also includes anovershoot-protection circuit 410, which is designed to preventfalse-positive detection of an open-circuit (OC) condition in the LED102(11).

The overshoot-protection circuit 410 has (i) a switch-drain node swdthat is connected to the drain of the main transistor MN1 of the switch112(11) and (ii) a switch-gate node swg that is connected to the gate ofMN1. Connected between nodes swd and swg are the series combination ofdiode D0 and transistor MN3 in parallel with the series combination ofdiode C0 and resistor R1, where the gate of MN3 is connected between C0and R1.

As in the conventional LED lighting system 100 of FIGS. 1 and 2, theswitch 112(11) is closed by turning off the turn-off transistor MN2 andclosing the turn-on switch S1 in order to turn on the main transistorMN1. When MN1 is fully on, the gate voltage swg is higher than the drainvoltage swd. If the diode DO were not present, then the integral bodydiode of transistor MN3 would conduct and prevent the gate of MN1 fromfully charging. The diode D0 blocks this reverse current path, therebyenabling the gate of MN1 to fully charge.

The switch 112(11) is opened by opening the turn-on switch S1 andturning on the turn-off transistor MN2 in order to turn off the maintransistor MN1. When the switch 112(11) is closed, current flows throughMN1 and the LED 102(11) is off. When the switch 112(11) is open, MN1 isoff and current flows through the LED 102(11), which is on.

FIG. 5 is a timing diagram showing certain signals associated with theoperation of the LED lighting system 400 of FIG. 4 for the same scenarioas in FIG. 3. The operation of the LED lighting system 400 from time t0up until time t2 are similar to the operation of the LED lighting system100 of FIGS. 1 and 2, with the switch 112(11) being open and the LED102(11) being on from time t0 to time t1, and the switch 112(11) beingclosed and the LED 102(11) being off from time t1 to time t2. At timet2, the switch 112(11) is again opened in order to turn on the LED102(11).

To transition from the LED 102(11) being off to the 102(11) being on, attime t2, the MLC chip 110 starts the process of turning off the maintransistor MN1 by opening the turn-on switch S1 and turning on theturn-off transistor MN2 (by driving MN2's gate voltage G2 high) in orderto drive the gate voltage swg of the main transistor MN1 low. When theparasitic inductances in the wires 104 u and 104 l resist theinstantaneous change in the current flowing through those wires due toMN1 beginning to turn off, the drain voltage Vd at the drain node swdwill rise, which will cause current to flow through capacitor C0 andresistor R1, which will result in a voltage across R1, which will applya positive gate voltage G3 to turn on the transistor MN3 just enough toallow some current to flow through diode D0 and transistor MN3, whichcurrent will fight against the drain current of the turn-off transistorMN2, which will slow down the turning off of the main transistor MN1,thereby allowing some current to continue to flow through MN1, whichwill prevent a large drain voltage swd and a corresponding largedrain-to-source voltage Vds from developing. As a result, the OCdetection circuit 118 in the MLC chip 110 will not misinterpret thenormal turning off of transistor MN1 as an OC condition of the LED102(11).

As current continues to flow through C0 and R1, C0 will charge and thatcurrent flow will decrease, thereby reducing the voltage across R1 andturning off MN3, which reduces the current flowing through D0 and MN3,which allows MN2 to turn off MN1, thereby completing the opening ofswitch 112(11) and the turning on of LED 102(11).

The sizes of the various components of the switch 112(11) and theovershoot-protection circuit 410 are selected to limit the rate ofchange of the drain voltage swd and thereby limit the rate of change ofthe drain-to-source voltage Vds in order to ensure that the magnitude ofMN1's drain-to-source voltage Vds stays below the threshold for OCdetection by the OC detection circuit 118, while still ensuring that theswitch 112(11) is opened sufficiently quickly.

As described above, when the switch 112(11) is open, the capacitor COgets charged with a positive voltage. When the switch 112(11) is closed,the switch-gate node swg is high and the switch-drain node swd is low,such that the capacitor CO will get charged with a negative voltage,thereby enabling current to flow through resistor R1 when the switch112(11) is again opened.

FIG. 6 is a schematic circuit diagram of overshoot-protection circuit610, according to an alternative embodiment of the present invention.The overshoot-protection circuit 610 is the same as theovershoot-protection circuit 410 of FIG, 4, with the addition of n-typetransistor MN4 and resistor R2, which are included to provide ESD(electro-static discharge) protection.

In particular, transistor MN4 acts as a Zener diode, and resistor R2limits the current through MN4 in case of an ESD pulse. Without R2, anESD pulse between the switch-drain node swd and the switch-gate node swgwould charge the gate of transistor MN3 to a high voltage, potentiallycausing permanent breakdown of its gate oxide.

Although the present invention has been described in the context of theovershoot-protection circuits 410 and 610 of FIGS. 4 and 6, thoseskilled in the art will understand that other circuit designs canachieve similar results. In general, overshoot-protection circuitry ofthe present invention limits the rate of change of the drain-to-sourcevoltage of the switch that controls an LED in order to preventfalse-positive detection of OC conditions of the LED.

Although the overshoot-protection circuits have been described as beingimplemented as part of an MLC chip, such as a modified version of theMLC chip 110 of FIG. 1, those skilled in the art will understand thatovershoot-protection circuits can alternatively be implemented externalto the MLC chip. In that case, overshoot-protection circuit of thepresent invention can be configured for use with a conventional MLCchip, such as the MLC chip 110 of FIG. 1.

Although the invention has been described in the context of circuitryimplemented using n-type transistors, those skilled in the art willunderstand that the invention can alternatively be implemented usingp-type switches either for the switches 112 of FIG. 4 or for theovershoot-protection circuitries 410 and 610 of FIGS. 4 and 6 or both.

Although the invention has been described in the context of a particularset of 12 LEDs 102, those skilled in the art will understand that theinvention can alternatively be implemented in the context of othernumbers of LEDs.

Although the invention has been described in the context of LED lightingsystems in which there is a one-to-one ratio between the switches (e.g.,switches 112) and LEDs (e.g., LEDs 102), the invention is not solimited. For example, the invention can be implemented in the context ofLED lighting systems in which at least one switch controls multiple LEDsthat are connected in series or in parallel.

In certain embodiments, the invention is an article of manufacturecomprising overshoot-protection circuitry for an LED (light-emittingdiode) controller. The overshoot-protection circuitry comprises (i) aswitch-drain node configured to be connected to a drain of a switch inthe LED controller that controls at least one LED, (ii) a switch-gatenode configured to be connected to a gate of the switch in the LEDcontroller that controls the LED, and (iii) inter-node circuitryconnected between the switch-drain and switch-gate node and configuredto limit magnitude of drain-to-source voltage of the switch.

In some embodiments, the inter-node circuitry is configured to limitrate of change of the drain-to-source voltage of the switch.

In some embodiments, the overshoot-protection circuitry comprises (i) adiode and a first transistor connected in series between theswitch-drain node and the switch-gate node and (ii) a capacitor and afirst resistor connected in series between the switch-drain node and theswitch-gate node, where the series connection of the capacitor and thefirst resistor is connected in parallel with the series connection ofthe diode and the first transistor; the capacitor is connected to thefirst resistor at a first internal node; and the first internal node isconnected to a gate of the first transistor.

In some embodiments, the overshoot-protection circuitry furthercomprises (i) a second resistor connected between the first internalnode and the gate of the first transistor and (ii) a second transistorconnected between the gate of the first transistor and the switch-gatenode, wherein a gate of the second transistor is connected to theswitch-gate node.

In some embodiments, the overshoot-protection circuitry is notintegrated with the LED controller on a single chip. In some embodimentsof the foregoing, the article further comprises the LED controller. Insome embodiments of the foregoing, the overshoot-protection circuitry isintegrated with the LED controller on a single chip.

In some embodiments, the article further comprises the LED. In someembodiments of the foregoing, the LED controller is a matrix lightingcontroller configured to control a plurality of LEDs. In someembodiments of the foregoing, the article further comprises theplurality of LEDs.

In some embodiments, the LED controller comprises open-circuit (OC)detection circuitry configured to monitor the magnitude of thedrain-to-source voltage of the switch in order to detect an OC conditionin the LED. By limiting the magnitude of the drain-to-source voltage ofthe switch, the overshoot-protection circuitry prevents false-positivedetection of an OC condition in the LED by the OC detection circuitry.

Unless explicitly stated otherwise, each numerical value and rangeshould be interpreted as being approximate as if the word “about” or“approximately” preceded the value or range.

For purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect,” “connecting,” or “connected” refer to any mannerknown in the art or later developed in which energy is allowed to betransferred between two or more elements, and the interposition of oneor more additional elements is contemplated, although not required.Conversely, the terms “directly coupled,” “directly connected,” etc.,imply the absence of such additional elements.

Signals and corresponding terminals, nodes, ports, or paths may bereferred to by the same name and are interchangeable for purposes here.

Transistor devices are typically shown as individual transistors forillustrative purposes. However, it is understood by those with skill inthe art that transistor devices will have various sizes (e.g., gatewidth and length) and characteristics (e.g., threshold voltage, gain,etc.) and may consist of multiple transistors coupled in parallel to getdesired electrical characteristics from the combination. Further, theillustrated transistor devices may be composite transistors.

As used in this specification and claims, the term “channel terminal”refers generically to either the source or drain of a MOSFET transistordevice. Similarly, as used in the claims, the terms “source,” “drain,”and “gate” should be understood to refer respectively either to thesource, drain, and gate of a MOSFET or to the emitter, collector, andbase of a bi-polar device if an embodiment of the invention isimplemented using bi-polar transistor technology.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain embodiments of this invention may bemade by those skilled in the art without departing from embodiments ofthe invention encompassed by the following claims.

In this specification including any claims, the term “each” may be usedto refer to one or more specified characteristics of a plurality ofpreviously recited elements or steps. When used with the open-ended term“comprising,” the recitation of the term “each” does not excludeadditional, unrecited elements or steps. Thus, it will be understoodthat an apparatus may have additional, unrecited elements and a methodmay have additional, unrecited steps, where the additional, unrecitedelements or steps do not have the one or more specified characteristics.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

The invention claimed is:
 1. An article of manufacture comprisingovershoot-protection circuitry for an LED (light-emitting diode)controller, the overshoot-protection circuitry comprising: aswitch-drain node configured to be connected to a drain of a switch inthe LED controller that controls at least one LED; a switch-gate nodeconfigured to be connected to a gate of the switch in the LED controllerthat controls the at least one LED; and inter-node circuitry connectedbetween the switch-drain and switch-gate nodes and configured to limit amagnitude of a drain-to-source voltage of the switch.
 2. The article ofclaim 1, wherein the inter-node circuitry is configured to limit a rateof change of the drain-to-source voltage of the switch, thereby limitingthe magnitude of the drain-to-source voltage of the switch.
 3. Thearticle of claim 1, wherein the overshoot-protection circuitrycomprises: a diode and a first transistor connected in series betweenthe switch-drain node and the switch-gate node; and a capacitor and afirst resistor connected in series between the switch-drain node and theswitch-gate node, wherein: the series connection of the capacitor andthe first resistor is connected in parallel with the series connectionof the diode and the first transistor; the capacitor is connected to thefirst resistor at a first internal node; and the first internal node isconnected to a gate of the first transistor.
 4. The article of claim 3,wherein the overshoot-protection circuitry further comprises: a secondresistor connected between the first internal node and the gate of thefirst transistor; and a second transistor connected between the gate ofthe first transistor and the switch-gate node, wherein a gate of thesecond transistor is connected to the switch-gate node.
 5. The articleof claim 1, wherein the overshoot-protection circuitry is not integratedwith the LED controller on a single chip.
 6. The article of claim 1,wherein the overshoot-protection circuitry is integrated with the LEDcontroller on a single chip.
 7. The article of claim 1, wherein thearticle further comprises the at least one LED.
 8. The article of claim7, wherein the LED controller is a matrix lighting controller configuredto control a plurality of LEDs.
 9. The article of claim 8, wherein thearticle further comprises the plurality of LEDs.
 10. The article ofclaim 1, wherein: the LED controller comprises open-circuit (OC)detection circuitry configured to monitor the magnitude of thedrain-to-source voltage of the switch in order to detect an OC conditionin the LED; and by limiting the magnitude of the drain-to-source voltageof the switch, the overshoot-protection circuitry preventsfalse-positive detection of an OC condition in the LED by the OCdetection circuitry.